In 1979, a few months after Margaret Thatcher moved into 10 Downing Street, Les Dawson turned up unannounced at a new club in Soho called the Comedy Store. He'd come to play an open spot. For the lucky few who were there that night, it was a fascinating encounter.

The Comedy Store had become a forum for a new kind of comedy and Dawson seemed to personify everything this revolt would sweep away. He'd learned his trade in working men's clubs, where he'd become the supreme purveyor of that old staple, the mother-in-law gag. Now in his late forties, he was a bastion of the showbiz establishment. By rights, he should have died on his proverbial arse. Instead, he stormed it. The reasons why he didn't die that night form the basis of Louis Barfe's conscientious, heartfelt book.

As Barfe tells us, Dawson wasn't remotely racist. He wasn't really sexist. But he was effortlessly funny, with a flair for bathetic wordplay worthy of P G Wodehouse. Who else could write about a snooty hotelier who looked down on his guests "with the disdain of a boiled owl"? Picture that hotelier and Basil Fawlty springs to mind, which is apt because John Cleese is one of a long list of classy collaborators whom Barfe cites to support his case that, beneath the gruff bluster, Dawson was a very clever comic. A superb pianist (like Tommy Cooper's conjuring, you have to know how to do it well to do it badly), above all he was a brilliant stand-up.

No one else could have come on at the Royal Variety Performance and opened with a line such as this one: "In 1645, Prince Rupert's mercenaries smashed Cromwell's left flank at Naseby; in 1871 the Franco-Prussian war ground to a halt at the siege of Metz; and, in 1903, from the Kyles of Bute, came the first report of an outbreak of sporran rash." Backstage, after the show, Dawson entered into a surreal debate with the Duke of Edinburgh - a firm fan - about whether black pudding should be fried or boiled.

Dawson wasn't always quite so smart, as Barfe's exhaustive trawl through his many sketch shows reminds us. On the one hand, a sketch co-starring Kenny Lynch is rightly praised as probably the only time Lynch appeared on television in the 1970s without reference to his skin colour. On the other, Barfe rightly criticises Dawson's penchant for limp-wristed stereotypes - notable even in an era of comedic mincing queens. Yet these aberrations were eclipsed by his sublime drag act with Roy Barraclough. As Barfe observes, "The women in Dawson's comedy are cartoonish, part McGill postcard, part Beano."

So if Dawson was that good, why didn't he enjoy an Indian summer, like other trad comics such as Bob Monkhouse? As Barfe quite correctly surmises, Dawson simply died too soon. The other thing that stood between him and true greatness, I reckon, was the bottle. Dawson could hold his drink but it can't have helped his stand-up. "Have you ever thought about trying, for once, just going out there and doing the show sober?" asked his producer Peter Whitmore. "What?" replied Dawson. "And face them alone?"

In the end, Dawson's greatest talent wasn't his gurning or his piano playing but his humanity. Having paid his dues in the pubs and clubs, he knew how much it cost to be a comic. After he ripped the Comedy Store apart that night, Dawson stuck around for a chat with those young pretenders. "There's one or two on here who'd get murdered up north," he said. "But it's useful, because it's a place to fail and no matter what type of comedian you want to be, you need the experience of failure."

A child of the Great Depression, Dawson had experienced his fair share of failure and that was what made him funny. His self-deprecating wit was honed by a long and arduous apprenticeship - Hoover salesman by day, struggling club comic by night. "Slumps don't bother me," he used to say. "I was a failure during the boom." In today's hard times, we could do with another comic like Dawson - someone who started out with nothing and who understood, as Eric Morecambe did, that the most important mantra for a comedian (or anyone else, for that matter) is: "Be kind."

William Cook's most recent book is "Kiss Me Chudleigh: the World According to Auberon Waugh" (Coronet, £19.99)